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This invention relates to a vehicle transmission control system, in particular a microprocessor-based electronic control system for a powershift transmission having hydraulic pressure operated transmission control elements, such as brakes and clutches, and solenoid operated valves for controlling the hydraulic pressure delivered to the control elements, and more particularly to a system and method for controlling the electrical signals applied to the solenoid operated valves.
Some manufacturers have used versions of electrohydraulic transmission controls with some success. For example, a powershift transmission sold by Ford New Holland, Ltd. has two modulating valves which control three different transmission control clutches. These valves are in turn controlled by an electronic controller. Such a proportional control allows a clutch element to be modulated during engagement and release of that element, and the controller provides the ability to vary the modulation for a particular element for each unique shift. Another system which includes on-off valves and at least one proportional control valve is described in U.S. Pat. No. 4,855,913, issued 8 Aug. 1989 to Brekkestran et al.
In such systems each clutch may have a different fill volume and thus may require different fill times. U.S. Pat. No. 5,082,097, issued on 21 Jan. 1992 to Goeckner et al. relates to a transmission controller for a transmission which includes a solenoid valve operated clutch and a solenoid valve for operating the clutch. The Goeckner et al. system also discloses a calibrating system or a system for determining a current signal corresponding to the point at which the clutch begins to transmit torque. U.S. Pat. No. 5,224,577 assigned to the Assignee of this application (attorney file 14018-US), discloses methods for determining a fill pressure calibration value and a fill volume calibration value.
It would be desirable to control the timing of subsequent pressurization of such a clutch (after it is filled) with respect to the end of the time period required to fill the clutch. If this is done, and if the fill time is adjusted to compensate for various conditions, then the timing of subsequent pressurization of the clutch will also be adjusted to compensate for the conditions. It would also be desirable, at a point just prior to the time when torque transmission is transferred from one element to another at the end of a shift, to control all elements with the same time base so that such torque transfers can be precisely timed relative to each other.